In the regulation of a synchronous motor, the position of the rotor is conventionally identified with a measuring absolute encoder, such as a resolver. The measuring accuracy of a resolver is extremely small. In addition, a resolver, as also other absolute sensors, must generally be installed on the shaft of the motor, which owing to the construction of the motor might be awkward. An absolute sensor installed on the shaft can also increase the axial length of the motor.
Recently, different sensorless identifications of position have also been developed, which are based on e.g. measurement of the inductance of the magnetic circuit of the motor as well as on estimation of the source voltage of the motor.
The basic principle in measurement of the inductance of a magnetic circuit is that an excitation is supplied to the motor as voltage pulses or current pulses and the response caused by them is measured to determine the inductance. The inductance varies as a function of the electrical angle at least to some extent, in which case the position between the rotor and the stator can be determined from the variation of the inductance. The determination of the inductance can be performed depending on the method for a rotor locked in its position or also for a rotating rotor.
In the aforementioned methods the accuracy of determining the position of the rotor varies. Especially in solid rotor electric motors the variation of the inductance is generally extremely small, in which case the inaccuracy of the determined position information increases. The variation in inductance is normally small also e.g. in the types of permanent-magnet motors in which the permanent magnets are fixed to the surface of the rotor as surface magnets.
For measuring inductance, the excitation signal supplied to the motor is summed with the actual supply voltage of the motor. That being the case the excitation signal reduces the maximum value of the amplitude of the supply voltage of the motor. This might lead to a reduction of the operating speed range of the motor, e.g. when supplying the motor with a frequency converter having an intermediate voltage circuit.
The source voltage of the motor can be estimated e.g. on the basis of the measurement of the stator current and of the stator voltage. The accuracy of the estimation of the source voltage decreases as the speed of the motor decreases, and estimation at zero speed is not normally possible.
Publication U.S. Pat. No. 5,057,759 discloses a determination of the rotor angle and the rotor angular velocity of the rotor of an alternating-current motor by means of a state observer.